1. Field of the Invention
This invention relates to a semiconductor laser device. This invention particularly relates to a semiconductor laser device for producing a laser beam, which may be utilized as stimulating rays for readout of a radiation image having been stored on a stimulable phosphor sheet. This invention also relates to a radiation image read-out method and apparatus using the semiconductor laser device.
2. Description of the Related Art
It has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a stimulable phosphor sheet, which comprises a substrate and a layer of the stimulable phosphor overlaid on the substrate. Stimulating rays are deflected and caused to scan pixels in the radiation image, which has been stored on the stimulable phosphor sheet, one after another. The stimulating rays cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted successively from the pixels in the radiation image having been stored on the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal by photoelectric read-out means. The stimulable phosphor sheet, from which the image signal has been detected, is then exposed to erasing light, and radiation energy remaining thereon is thereby released.
As radiation image read-out apparatuses for use in the radiation image recording and reproducing systems described above, besides conventional point scanning types of radiation image read-out apparatuses, line scanning types of radiation image read-out apparatuses have heretofore been used. The line scanning types of radiation image read-out apparatuses have been proposed from the point of view of keeping the emitted light detection time short, reducing the size of the apparatus, and keeping the cost low. With the point scanning types of the radiation image read-out apparatuses, in cases where, for example, a laser beam is utilized as the stimulating rays, the laser beam is radiated out from a semiconductor laser device, and an image of the laser beam is formed on a stimulable phosphor sheet by use of an image forming optical system. Also, the laser beam is caused by a rotating polygon mirror, or the like, to scan in a main scanning direction, and sites (points) on an entire area of a surface of the stimulable phosphor sheet are thus scanned and stimulated successively by a spot of the laser beam. Further, the light, which has been emitted from each of the stimulated sites (points) on the entire area of the surface of the stimulable phosphor sheet, is detected.
With the line scanning types of the radiation image read-out apparatuses, a line light source for irradiating linear stimulating rays onto a stimulable phosphor sheet is utilized as a stimulating ray source, and a line sensor comprising a plurality of photoelectric conversion devices arrayed along the length direction of a linear area of the stimulable phosphor sheet, onto which linear area the stimulating rays are irradiated by the line light source, is utilized as photoelectric read-out means. (The length direction of the linear area of the stimulable phosphor sheet will hereinbelow be referred to as the main scanning direction.) Also, the line scanning types of the radiation image read-out apparatuses comprise scanning means for moving the stimulable phosphor sheet with respect to the line light source and the line sensor and in a direction, which is approximately normal to the main scanning direction. (The direction, which is approximately normal to the main scanning direction, will hereinbelow be referred to as the sub-scanning direction.) (The line scanning types of the radiation image read-out apparatuses are described in, for example, U.S. Pat. No. 4,922,103.) An example of the line light source for irradiating, for example, a linear laser beam as the linear stimulating rays onto the stimulable phosphor sheet comprises a semiconductor laser device and an optical system for expanding the laser beam, which has been radiated out from the semiconductor laser device, into the linear laser beam extending in the main scanning direction on the stimulable phosphor sheet.
In cases where a radiation image having been stored on a stimulable phosphor sheet is read out by use of the line scanning types of the radiation image read-out apparatuses described above, shading compensation is often performed in order to compensate for, for example, nonuniformity in intensity of light irradiated to the stimulable phosphor sheet along the main scanning direction. The shading compensation is performed by use of a shading compensation signal. The shading compensation signal is formed such that a signal having been obtained from operations, wherein radiation is uniformly irradiated to the entire area of the surface of a stimulable phosphor sheet, and wherein energy having thus been stored on the uniformly exposed stimulable phosphor sheet is read out to yield the signal, may be rendered uniform.
Ordinarily, the semiconductor laser devices for producing the laser beam comprise a semiconductor laser chip for producing the laser beam and a combination of a casing and a cap for enclosing the semiconductor laser chip. (The combination of the casing and the cap for enclosing the semiconductor laser chip is referred to as a package.) The package is provided with a radiating window, through which the laser beam is capable of passing. The laser beam having been produced by the semiconductor laser chip passes through the radiating window of the package and is thus radiated out from the package. Such that return light may not impinge upon the semiconductor laser chip, the radiating windows of the semiconductor laser devices are ordinarily coated with an anti-reflection film.
The semiconductor laser devices have heretofore been utilized in various fields. In particular, recently, optical recording media (i.e., optical disks), such as compact disks (CD's), minidisks (MD's), and digital video disks (DVD's), rapidly became popular. The semiconductor laser devices became essential to recording apparatuses for recording digital information on the optical recording media and for reproducing apparatuses for reproducing the digital information from the optical recording media. Also, a wavelength of a produced laser beam, which is utilized in the recording apparatuses and the reproducing apparatuses, varies for different kinds of optical disks. For example, a laser beam having a wavelength of 780 nm is utilized for the reproduction of the digital information from the CD's. Also, a laser beam having a wavelength of 650 nm is utilized for the reproduction of the digital information from the DVD'S. Manufacturers for the semiconductor laser devices produce the semiconductor laser devices for producing laser beams having various different wavelengths in accordance with various different applications. In order for the production cost to be kept low, each of the radiating windows of the packages of the semiconductor laser devices is ordinarily coated with an anti-reflection film, which is capable of coping with laser beams having wavelengths falling within a wide wavelength range, such that the radiating window is capable of being used for various semiconductor laser chips which produce the laser beams having different wavelengths.
Recently, in order for the information recording to be performed at a high recording density, the semiconductor laser devices for the optical recording media described above tend to be designed so as to produce laser beams having short wavelengths (e.g., wavelengths of at most 700 nm), which wavelengths are appropriate as the wavelengths of the stimulating rays for the stimulable phosphor sheets. As the semiconductor laser devices to be used as the stimulating ray sources in the radiation image read-out apparatuses, the semiconductor laser devices, which are capable of producing the laser beams having the wavelengths appropriate as the wavelengths of the stimulating rays for the stimulable phosphor sheets, are selected from the semiconductor laser devices described above.
The anti-reflection film, which has been coated on each of the radiating windows of the semiconductor laser devices utilized in the recording apparatuses and the reproducing apparatuses for the optical recording media described above, has the anti-reflection effects with respect to the laser beams having wavelengths falling within a wide wavelength range. However, the anti-reflection film, which has been coated on each of the radiating windows of the semiconductor laser devices utilized in the recording apparatuses and the reproducing apparatuses for the optical recording media, does not have uniform anti-reflection effects with respect to all of the laser beams having various different wavelengths. FIG. 4 is a graph showing characteristics of an example of an anti-reflection film having been coated on a radiating window of a semiconductor laser device of the type described above. As illustrated in FIG. 4, the anti-reflection film has low reflectivities with respect to laser beams having wavelengths falling within a range of approximately 400 nm to approximately 900 nm. The anti-reflection film has the lowest reflectivity of approximately 1% with respect to the laser beam having a wavelength of 780 nm. However, the anti-reflection film has the reflectivities of as high as several percent with respect to the laser beams having other wavelengths. For example, with respect to the laser beam having a wavelength of 660 nm, the anti-reflection film has the reflectivity of as high as approximately 3%.
FIG. 5A is a graph showing a far field pattern (FFP) of a laser beam having been produced by a semiconductor laser chip, which FFP is obtained in cases where an anti-reflection film having been coated on a radiating window of a semiconductor laser device has a reflectivity of 3% with respect to the laser beam having been produced by the semiconductor laser chip. The FFP illustrated in FIG. 5A has been obtained from measurement made at a temperature of 20° C. Approximately identical measurement results have also been obtained from measurements made at different temperatures, e.g. 50° C.
In FIG. 5A, the solid line indicates the FFP in a maximum radius direction of a cross-section of the laser beam having been produced by the semiconductor laser chip, which cross-section is normal to the laser beam radiating direction. (The maximum radius direction of the cross-section of the laser beam will hereinbelow be referred to as the V direction.) Also, in FIG. 5A, the broken line indicates the FFP in a direction, which is normal to the maximum radius direction, i.e. the V direction, of the cross-section of the laser beam. (The direction, which is normal to the maximum radius direction, i.e. the V direction, of the cross-section of the laser beam, will hereinbelow be referred to as the H direction.) As illustrated in FIG. 5A, in the H direction, little disturbance occurs with the distribution of the intensity of the laser beam. However, in the V direction, a disturbance occurs with the distribution of the intensity of the laser beam, and the profile of the laser beam is deformed. The disturbance of the distribution of the intensity of the laser beam occurs due to interference, which occurs at the entry surface and the radiating surface of the radiating window due to reflection from the anti-reflection film, and return light toward the semiconductor laser chip.
In the cases of the recording apparatuses and the reproducing apparatuses for the optical recording media, slight deformation of the profile of the laser beam has little adverse effect upon the information recording and the information reproduction. Also, in the cases of the point scanning types of the radiation image read-out apparatuses, slight deformation of the profile of the laser beam has little adverse effect upon the radiation image read-out operation. However, in the cases of the line scanning types of the radiation image read-out apparatuses, in which the semiconductor laser devices described above are utilized as the stimulating ray sources, the laser beam having been produced as the stimulating rays by the semiconductor laser device is expanded into a linear laser beam extending in the main scanning direction. Therefore, in cases where the profile of the laser beam is deformed in the V direction described above, the distribution of the intensity of the stimulating rays in the main scanning direction on the stimulable phosphor sheet becomes nonuniform. As a result, the problems occur in that defects in image quality, such as vertical streak-like nonuniformity, arise in the radiation image reproduced from the image signal obtained from the radiation image read-out operation.
Also, in cases where the shading compensation is to be performed in order to compensate for nonuniformity in intensity of the stimulating rays irradiated to the stimulable phosphor sheet along the main scanning direction, it is necessary for the shading compensation signal to be formed. Such that good results of compensation may be obtained by suppressing adverse effects of variations with the passage of time, it is necessary for the shading compensation signal to be updated immediately before the image read-out operation is performed. However, currently, in order for the shading compensation signal to be updated, it is necessary to perform the operation for uniformly irradiating the radiation to the stimulable phosphor sheet. Therefore, ordinarily, the updating of the shading compensation signal is performed by experts only at the time of limited occasions, such as delivery of the radiation image read-out apparatuses from factories, changeover of apparatus members (e.g., a scanner head), or changeover of the stimulable phosphor sheets. Accordingly, currently, with the technique for performing the shading compensation, it is not always possible to cope with variations with the passage of time.